jigsaw toolbox blocks should be 'deletable' so that they can be put back...

... in the toolbox

While working on the blockly-core fix, I noticed I can't drag a block out of the toolbox and put it back in (something you're meant to be able to do when concrete blocks is enabled - currently only in jigsaw).

This makes those blocks deletable, which really means deletable from the mainWorkspace, which puts them back into the toolbox.